Evolution of the Astonishing Naica Giant Crystals in Chihuahua, Mexico

Iván Carreño-Márquez,Bernardo Pérez-Cázares,Luis Fuentes-Cobas,Isaí Castillo-Sandoval,Hilda Esparza-Ponce,María Montero-Cabrera,María Fuentes-Montero,Esperanza Menéndez-Méndez

doi:10.3390/min11030292

Abstract

Calcium sulfate (CaSO4) is one of the most common evaporites found in the earth’s crust. It can be found as four main variations: gypsum (CaSO4∙2H2O), bassanite (CaSO4∙0.5H2O), soluble anhydrite, and insoluble anhydrite (CaSO4), being the key difference the hydration state of the sulfate mineral. Naica giant crystals’ growth starts from a supersaturated solution in a delicate thermodynamic balance close to equilibrium, where gypsum can form nanocrystals able to grow up to 11–12 m long. The growth rates are reported to be as slow as (1.4 ± 0.2) × 10−5 nm/s, taking thousands of years to form crystals with a unique smoothness and diaphaneity, which may or may not include solid or liquid inclusions. Conservation efforts can be traced back to other gypsum structures found prior to Naica’s. Furthermore, in the last two decades, several authors have explored the unique requirements in which these crystals grow, the characterization of their environment and microclimatic conditions, and the prediction of deterioration scenarios. We present a state-of-the-art review on the mentioned topics. Beyond the findings on the origin, in this work we present the current state and the foreseeable future of these astounding crystals.

Highlights

Giant crystals are rare spectacles and can only be observed in unique places around the globe
Gypsum crystals of exceptional size can be found in the Debar mine (Macedonia), which houses gypsum crystals up to seven meters long in caves near the surface
CaSO4·0.5H2O crystallizes in a hexagonal prism form, which is topped by rhombohedral faces, epitaxy on a gypsum substrate is relatively standard

Summary

Introduction

Giant crystals are rare spectacles and can only be observed in unique places around the globe. Gypsum crystals of exceptional size can be found in the Debar mine (Macedonia), which houses gypsum crystals up to seven meters long in caves near the surface They remain at room temperature and are surrounded by hydrothermal puddles [7]. Trace elements and isotope studies in “Cueva de los cristales” speleothems suggest that water circulation has varied between the last glaciation, 57 ka ago, and the Holocene in periods from 14.5 ± 4 to 7.9 ± 0.1 and after 7.9 ± 0.1 ka [23] The study of these speleothems via U-Th dating allows estimating the phreatic level oscillations during the growth of the crystals [30]. In January 2015, a rise in the phreatic level stopped exploration and exploitation of the mine

Crystals

Structure

Nucleation and Growth

Final Remarks